if it's intensifying/fading every turn of the roll then it might be temperature damage or pressure sensitization. if a roll is stored for example in the fridge in a way that one side of the roll is much warmer than then other and it's like that for long time then it may do this kind of damage: other side of the roll ages faster than the other. I have had this kind of problem in some eBay rolls but otherwise not that common. I rarely store rolls for more than two years after purchase though so if yours was very old it is easier for it to generate this kind of problem.

X-ray damage is normally faster pulsing which may "sweep" across the image as far as I have seen